When depositing an amorphous thin film at low temperature (lower than 300 degrees C.), the surface roughness deteriorates rapidly if doping boron. Particularly, if the target thickness of the amorphous thin film is 200 Å, it may be difficult to forming the amorphous thin film having a surface roughness (RMS) equal to or less than 0.3 nm. Therefore, the technology for improving the surface roughness is needed.